Press-on pedicle screw assembly

ABSTRACT

A tulip assembly configured to be coupled to a screw head having a first diameter includes a housing, wherein the housing defines an inner diameter configured to engage the first diameter of the screw head. The exemplary inner diameter of the housing being smaller than the first diameter of the screw head.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 11/952,709 filed Dec. 7, 2007 which claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Patent Application No: 60/873,698, filed Dec. 7, 2006, titled “Press on Screw Tulip,” which applications are incorporated herein by reference in their entirety.

TECHNICAL FIELD 1. Field of the Invention

The present exemplary system and method relates to medical devices. More particularly, the present exemplary system and method relates to percutaneous orthopedic rod placement devices.

2. Background and Related Art

The use of bone stabilization/fixation devices to align or position bones is well established. Furthermore, the use of spinal bone stabilization/fixation devices to align or position specific vertebrae or a region of the spine is well established. Typically such devices for the spine utilize a spinal fixation element, comprised of a relatively rigid member such as a plate or a rod that is used as a coupler between adjacent vertebrae. Such a spinal fixation element can effect a rigid positioning of adjacent vertebrae when attached to the pedicle portion of the vertebrae using pedicle bone anchorage screws. Once the coupled vertebrae are spatially fixed in position, procedures can be performed, healing can proceed, or spinal fusion may take place.

Spinal fixation elements may be introduced to stabilize the various vertebrae of the spine. Some devices for this purpose are designed to be attached directly to the spine, but the generally invasive nature of standard paraspinal approach used to implant these devices may pose drawbacks. For example, muscle disruption and blood loss may result from standard paraspinal implantation approaches.

Conventional pedicle screw systems and even more recently designed pedicle screw systems also have several drawbacks. Some of these pedicle screw systems are rather large and bulky, which may result in more tissue damage in and around the surgical site when the pedicle screw system is installed during surgery. The prior art pedicle screw systems have a rod-receiving device that is pre-operatively coupled or attached to the pedicle screw. In addition, some of the prior art pedicle screw systems include numerous components that must all be carefully assembled together. Further, traditional pedicle screw systems are pre-operatively assembled, which makes these systems more difficult to install and maneuver in a spinal operation where MIS techniques are used.

BRIEF SUMMARY OF THE INVENTION

A tulip assembly configured to be coupled to a screw head having a first diameter includes a housing, wherein the housing defines an inner diameter configured to engage the first diameter of the screw head. The exemplary inner diameter of the housing being smaller than the first diameter of the screw head.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate various embodiments of the present system and method and are a part of the specification. Together with the following description, the drawings demonstrate and explain the principles of the present system and method. The illustrated embodiments are examples of the present system and method and do not limit the scope thereof.

FIG. 1 is an assembled perspective view of a press-on pedicle screw system, according to one exemplary embodiment.

FIG. 2 is a partial side unassembled view of a press-on pedicle screw system, according to one exemplary embodiment.

FIG. 3 is a side view of a pedicle screw, according to one exemplary embodiment.

FIG. 4 is a side partial cut-away view of a tulip and rod assembly of the press-on pedicle screw system of FIG. 1, according to one exemplary embodiment.

FIG. 5 is an exploded partial cut-away side view of the tulip assembly of FIG. 1, according to one exemplary embodiment.

FIG. 6 is an assembled partial cut-away side view of the pedicle screw system of FIG. 1, according to one exemplary embodiment.

FIGS. 7A and 7B are cross-sectional side-views of a press-on tulip bodies, according to various exemplary embodiments.

FIG. 8 is a partially exploded perspective view and of an alternative press-on pedicle screw system, according to one exemplary embodiment.

FIG. 9 is a cross-sectional partially exploded side view of the alternative press-on pedicle screw system of FIG. 8, according to one exemplary embodiment.

FIG. 10 is a flow chart illustrating a method for inserting a press-on tulip assembly on a pedicle screw, according to one exemplary embodiment.

Throughout the drawings, identical reference numbers designate similar but not necessarily identical elements.

DETAILED DESCRIPTION OF THE INVENTION

The present specification describes a system and a method for securely coupling a tulip assembly relative to a pedicle screw via an interference fit. Further, according to one exemplary embodiment, the present specification describes the structure of a tulip assembly configured to be placed on the head of a pedicle screw after placement of the pedicle screw in a patient's body and configured to receive and positionally secure a top loaded rod. Further details of the present exemplary system and method will be provided below.

By way of example, pedicle screw systems may be fixed in the spine in a posterior lumbar fusion process via minimally invasive surgery (MIS) techniques. The systems are inserted into the pedicles of the spine and then interconnected with rods to manipulate (e.g., correct the curvature, compress or expand, and/or structurally reinforce) at least portions of the spine. Using the MIS approach to spinal fixation and/or correction surgery has been shown to decrease a patient's recovery time and reduce the risks of follow-up surgeries.

The ability to efficiently perform spinal fixation and/or correction surgeries using MIS techniques is enhanced by the use of pedicle screw systems provided in accordance with the present exemplary systems and methods, which systems and methods provide a number of advantages over conventional systems. For example, a pedicle screw system in accordance with one embodiment of the present exemplary system and method provides the advantage that the pedicle screw may be inserted into the bone without being pre-operatively coupled with the rod-coupling assembly (hereinafter referred to as a tulip assembly). This is advantageous because the surgeon often needs to do other inter-body work after inserting the pedicle screw, but before attaching the tulip assembly. Such an advantageous pedicle screw system may be even more crucial when using MIS techniques because the inter-body spatial boundaries in which the surgeon must work may be quite limited.

In addition, pedicle screw systems in accordance with several embodiments of the present system and method advantageously reduce the overall size and height of the pedicle screw system when compared to traditional pedicle screw systems. Reduced size and height of the pedicle screw system reduces tissue damage surrounding a surgical site and may also increase a patient's relative comfort.

The term “distraction,” when used herein and when used in a medical sense, generally relates to joint surfaces and suggests that the joint surfaces move perpendicular to one another. However when “traction” and/or “distraction” is performed, for example on spinal sections, the spinal sections may move relative to one another through a combination of distraction and gliding, and/or other degrees of freedom.

Another advantageous feature of at least one embodiment of the present exemplary system and method is that an all-inclusive tulip assembly that can be coupled to the head portion of the pedicle screw intra-operatively is disclosed. This advantageous tulip assembly may include the aspects or features that enable the tulip assembly to be locked onto the head portion of the pedicle screw. The present exemplary tulip assembly may decrease the complexity of the pedicle screw system installation while simultaneously reducing the overall size of the pedicle screw system.

In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present system and method for providing a press-on screw assembly configured to be coupled to the head of a spinal fastener via a compression fit. It will be apparent, however, to one skilled in the art that the present method may be practiced without these specific details. Reference in the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. The appearance of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment.

Exemplary Overall Structure

While the present system and method may be practiced by any number of bone fixation systems, a first exemplary embodiment of the present system and method will be described herein, for ease of explanation only, in the context of a pedicle screw system. Accordingly, the present system and method includes, according to one exemplary embodiment illustrated in FIG. 1, a pedicle screw system (100) including a pedicle screw (102), a rod member (104), and at least one coupling assemblies (106), herein after referred to as a tulip assembly (106). According to one exemplary embodiment of the present system and method, the tulip assembly (106) is configured to receive and securely position the rod member (104). Alternatively, as illustrated in FIG. 1, the at least one tulip assembly (106) may be securely and permanently coupled to the rod member (104) at a coupling site (107) via any number of coupling means, as will be described in detail below. According to the present exemplary system and method, the at least one tulip assembly (106) is configured to be securely coupled to the pedicle screw (102) via an interference fit. Operation of the tulip assembly (106) as well as its interaction with both the pedicle screw (102) and the rod (104) will be described in further detail below with reference to the Figures.

According to one exemplary embodiment, FIG. 1 generally shows a pedicle screw system (100) comprising a pedicle screw (102), a rod member (104), and a coupling assembly (106), hereinafter referred to as a tulip assembly (106). As illustrated in FIG. 1, the pedicle screw system (100) is configured to securely couple the tulip assembly (106) to the pedicle screw (102), thereby locking or fixing the tulip assembly (106) in a relative angular position relative to the pedicle screw (102). Additionally, as shown in FIG. 1, the present exemplary pedicle screw system (100) is devoid of separate pedicle screw coupling features and therefore is configured to couple the head of a pedicle screw while minimizing the outer diameter size of the tulip assembly (106).

FIG. 2 illustrates an unassembled exploded side view of the present pedicle screw system (100), according to one exemplary embodiment. As illustrated in FIG. 2, the coupling assembly or tulip assembly (106) of the pedicle screw system (100) includes an engagement orifice (200) configured to engage the head portion (110) of the pedicle screw (102), as described in further detail below. Detailed descriptions of each component of the present pedicle screw system (100), along with a number of alternative embodiments will be described in further detail below, with reference to FIGS. 2 through 9.

FIG. 3 further illustrates the components of a pedicle screw (102) that may be used in connection with the present exemplary system and method, according to one exemplary embodiment. As illustrated in FIG. 3, the pedicle screw (102) includes an elongated, threaded portion (108) and a head portion (110). Although pedicle screws (102) are generally known in the art, the head portions (110) may be of varying configurations depending on what type of tulip assembly (160) is to be coupled to the pedicle screw (102). The head portion (110) of the present exemplary pedicle screw (102) includes a driving feature (124) and a maximum diameter portion (126). The driving feature (124) of the present exemplary pedicle screw (102) permits the screw to be inserted into a pedicle bone and/or other bone. According to one exemplary embodiment, the pedicle bone is a part of a vertebra that connects the lamina with a vertebral body. Additionally, according to the present exemplary embodiment, the driving feature (124) can be used to adjust the pedicle screw (102) prior to or after the tulip assembly (106) is coupled to the pedicle screw (102). In the illustrated embodiment, the head portion (110) of the pedicle screw (102) is coupled to the threaded portion (108) and includes a generally spherical surface (127) with a truncated or flat top surface (128).

In one exemplary embodiment, the pedicle screw (102) is cannulated, which means a channel (130) (shown in dashed lines and extending axially through the pedicle screw (102)) extends through the entire length of the pedicle screw (102). The channel (130) allows the pedicle screw (102) to be maneuvered over and receive a Kirschner wire, commonly referred to as a K-wire. The K-wire is typically pre-positioned using imaging techniques, for example, fluoroscopy imaging, and then used to provide precise placement of the pedicle screw (102). While the pedicle screw (102) illustrated in FIG. 3 includes a number of components, a number of variations may be made including, but in no way limited to, varying the type of driving feature (124), varying the shape and/or size of the head portion, varying materials, varying lengths, and the like.

Returning again to FIG. 1, the pedicle screw system includes a tulip assembly (106) configured to be locked to the head portion (110) of a pedicle screw (102) via an interference fit. FIGS. 4, 5, and 6 illustrate the various components of the present exemplary tulip assembly (106), according to one exemplary embodiment. FIG. 4 illustrates a partially cut-away view of a tulip assembly (106), according to one exemplary embodiment. As illustrated in FIG. 4, the present exemplary tulip assembly (106) includes a generally cylindrical tulip body (450) defining an engagement orifice (200). As shown in FIG. 4, the engagement orifice (200) defined by the generally cylindrical tulip body (450) includes a screw head contact surface (410) forming an inner wall portion of the engagement orifice (200). According to one exemplary embodiment, when the present exemplary tulip assembly is coupled to the head portion (110) of a pedicle screw (102), the screw head contact surface (410) of the tulip assembly is sized to contact and securely couple the head portion of the pedicle screw via an interference fit.

Additionally, according to the exemplary embodiment illustrated in FIG. 4, an upper bore (400) is formed in the center of the tulip assembly (106) to provide access to and through the engagement orifice (200), thereby providing top access to a driving feature (124; FIG. 3) of a pedicle screw (102; FIG. 3) and/or a K-wire. While on exemplary engagement orifice (200) including an upper bore (400) is illustrated in FIG. 4, any number of geometries may be used to define the engagement orifice (200) according to the teachings of the present exemplary system and method.

FIG. 5 further illustrates an exemplary configuration for practicing the present exemplary system and method. As shown in FIG. 5, the present exemplary pedicle screw system (100) includes a rod member (104) permanently and securely coupled to a plurality of tulip assemblies (106). By permanently coupling the rod member (104) to the outer housing of the tulip assemblies (106), rod binding mechanisms are unnecessary in the formation of the tulip assemblies (106). Consequently, the outer diameter of the resulting pedicle screw system may be minimized, thereby reducing trauma and tissue damage to a patient. According to one exemplary embodiment, the rod member (104) may be coupled to the outer surface of the tulip assemblies (106) by any number of methods including, but in no way limited to, welding, forming the system from a single piece of material, adhesives, mechanical fasteners, and the like.

As mentioned previously, each of the tulip assemblies (106) includes an engagement orifice (200) including a screw head contact surface (410). According to one exemplary embodiment, the screw head contact surface (410) defines an internal diameter (500) of the tulip assembly (106). Additionally, as illustrated in FIG. 5, the screw head (110) of the exemplary pedicle screw (102) has a maximum screw head diameter (550). According to one exemplary embodiment of the present exemplary system and method, the internal diameter (500) defined by the screw head contact surface (410) is smaller than the maximum screw head diameter (550) of the screw head (110) of the exemplary pedicle screw (102), such that when the screw head of the exemplary pedicle screw (102) is pressed into the engagement orifice (200), the screw head and the screw head contact surface combine to create a press fit or an interference fit.

As used in the present specification, and in the appended claims, the term “interference fit” shall be interpreted broadly as including the joining of any two mating parts such that one or the other (or both) parts slightly deviate in size from their nominal dimension, thereby deforming each part slightly, each being compressed, the interface between two parts creating a union of extremely high friction. The word interference refers to the fact that one part slightly interferes with the space that the other is occupying in its nominal dimension.

According to one exemplary embodiment, the allowance, or planned difference from nominal size of both the internal diameter (500) defined by the screw head contact surface (410) is smaller than the maximum screw head diameter (550) of the screw head (110) is sufficiently large that the force sufficient to overcome the resulting interference fit between the screw head contact surface (410) and the screw head (110) is larger than a force sufficient to pull the screw (102) out of a bone, or approximately 200 inch pounds.

FIG. 6 illustrates the engagement of a tulip assembly (106) on a pedicle screw (102), according to one exemplary embodiment. As illustrated in FIG. 6, to engage the tulip assembly (106) to the screw head portion (110) of the pedicle screw (102), an instrument (not shown) can be coupled to the underside of the screw head (110) and to the top of the tulip assembly (106). Once positioned, a load (F) can be applied to the top of the tulip assembly (106) to press the screw head (110) into the engagement orifice (200). Similarly, disassembly can be achieved by pulling up on the tulip assembly (106) while driving an instrument through the upper bore (400) of the tulip assembly (106) to disengage the screw head (110) from the engagement orifice (200).

According to one exemplary embodiment, the desired interference fit sufficient to create a removal force of at least approximately 200 inch pounds may be dependent upon the materials used to form the screw head (110) and/or the screw head contact surface (410). Particularly, according to one exemplary embodiment, the screw head (110) and/or the screw head contact surface (410) is formed of one of a titanium, a cobalt chrome, and/or a nitinol (nickel titanium) based material. According to one exemplary embodiment, titanium, cobalt chrome, and/or nitinol, allow for a small amount of deformation without yielding. Consequently, as illustrated above, the difference in the maximum screw head diameter (550) of the screw head (110) and the internal diameter (500) defined by the screw head contact surface (410) is sufficient to provide an interference fit between the screw head and the screw head receptacle, without yielding the material of either component. Additionally, according to one exemplary embodiment, the interference fit is sufficiently strong to prevent the receptacle from being pulled off of the screw head, i.e., has a removal force of at least approximately 200 inch pounds.

According to one exemplary embodiment, the entire tulip assembly (106) may be formed of a known material such that the dimensions desired to create the interference fit can be readily calculated. Alternatively, as illustrated in FIG. 7A, one or more inserts (710) may be formed in the engagement orifice (200) to provide a screw head contact surface (410) with known properties, while not requiring that the remaining tulip assembly exhibit those same qualities. The placement of the one or more inserts (710) may be performed using any number of mechanical and/or chemical methods including, but in no way limited to, welding, using adhesives, material deposition methods, and the like.

While the interference fit provided above is described as occurring with only elastic deformation, the present system and method may be performed with slightly looser tolerances of the internal diameter (500) defined by the screw head contact surface (410) and the maximum screw head diameter (550) of the screw head (110) such that some plastic deformation occurs, so long as the removal force of the tulip assemblies (106) from the screw head (110) is sufficiently strong to prevent the receptacle from being pulled off of the screw head, i.e., has a removal force of at least approximately 200 inch pounds.

Furthermore, according to one exemplary embodiment, the surface defined by the screw head contact surface (410) has a constant diameter. Consequently, a single interference fit is achieved when engaged with the screw head (110). That is, according to one exemplary embodiment, the removal force will be substantially the same regardless of the level of engagement between the screw head contact surface (410) and the screw head (110).

Alternatively, according to one exemplary embodiment illustrated in FIG. 7B, the internal diameter (500) defined by the screw head contact surface (410) may vary, providing a potential for differing levels of engagement between the screw head contact surface (410) and the screw head (110). Particularly, according to one exemplary embodiment, the tulip assembly (106) may have a first (750) and a second (755) internal diameter (500), both smaller than the maximum screw head diameter (550) of the screw head (110). According to this exemplary embodiment, a first larger diameter (750) may be engaged or press fit to the head (110) of the pedicle screw to form a provisional lock, thereby establishing the engagement of the tulip assembly while providing some resisted movement. Once a desired assembly and operation have been performed, the second smaller diameter (755) may be engaged with the maximum screw head diameter (550) of the screw head (110) to establish the final lock sufficient to maintain engagement.

While the above-mentioned exemplary embodiments are described as having the rod member (104) permanently coupled to the outer surface of the tulip assembly (106), thereby reducing the number of parts and the size of the pedicle screw system (100), any number of rod retention members may be formed in the upper portion of the tulip assembly to retain the rod member (104). According to one exemplary embodiment illustrated in FIGS. 8 and 9, an exemplary pedicle screw system (800) includes a tulip member (806), having an upper portion and a lower portion. According to the illustrated exemplary embodiment, the lower portion of the tulip member (806) includes an engagement orifice (200) to provide a screw head contact surface (410) as described above. However, in contrast to the tulip members described previously, the upper portion of the exemplary tulip member (806) illustrated in FIG. 8 includes a threaded inner wall (810) configured to receive a set screw member (808). Additionally, according to one exemplary embodiment, the exemplary tulip member (806) includes an orifice (900) passing through the walls of the exemplary tulip member (806). According to one exemplary embodiment, the orifice is sufficiently sized to allow for the reception and translation of the rod member (104) there through. As the rod member (104) is passed through the orifice (900), the set screw (808) may be advanced along the threaded inner wall (810) of the exemplary tulip member (806) until it engages the rod member (104) and imparts a retaining force thereon.

Exemplary Implementation and Operation

FIG. 10 illustrates one exemplary method for utilizing the present exemplary press-on pedicle screw system, according to one exemplary embodiment. As illustrated in FIG. 10, the exemplary method may begin by first inserting screws into a desired bone of a patient's spine (step 1010). The placement and/or number of pedicle screw systems (100) to be used in a patient may be pre-operatively determined based on a pre-operative examination of the patient's spinal system using non-invasive imaging techniques known in the art, such as x-ray imaging, magnetic resonance imaging (MRI), and/or fluoroscopy imaging, for example. The pedicle screws (102) may be maneuverable to achieve a desired placement, orientation, and/or angular position.

Once the desired pedicle screw (102) is properly placed (step 1010), a tulip member (806) may be inserted and guided to the previously placed screw (step 1020). According to one exemplary embodiment, the placement of the tulip member (806) may be facilitated by the use of a Kirschner wire or non-invasive imaging methods including, but in no way limited to, x-ray imaging, magnetic resonance imaging (MRI), and/or fluoroscopy imaging, for example.

Once the tulip assembly (806) is at the desired position relative to the pedicle screw (102), the tulip assembly (806) can be fixed or locked onto the pedicle screw (102) by a downward force as described above. According to one exemplary embodiment, the tulip assembly (806) is fixed onto the pedicle screw (102), at least provisionally (step 1040), before the rod member (104) is inserted into the tulip assembly (802) (step 1030). In another embodiment, as illustrated in FIG. 10, the tulip assembly (806) is fixed onto the pedicle screw (102) contemporaneously as the rod is inserted into the tulip assembly (806), or even after the rod member (104) is inserted into the tulip assembly.

Once the tulip assembly (806) is provisionally retained on the screw (102), the rod (104) may be retained in the tulip assembly (806). According to one exemplary embodiment, the rod member (104) may be captured by the tulip assembly (806) by advancing the set screw (808) along the threaded inner wall (810) of the exemplary tulip member (806) until the set screw engages the rod member (104) and imparts a retaining force thereon. With the rod member (104) correctly positioned and any positional manipulation procedures completed, the tulip assembly (806) can be further advanced along the engagement orifice (200) to provide a final positional and angular lock of the tulip assembly relative to the screw (step 1060).

While the above-mentioned method is described and illustrated as a step-wise process, it will be understood that the illustrated order of performing the detailed steps is not necessary. Rather, the steps of the exemplary method illustrated herein may be performed in any number of combinations to practice the present exemplary system and method. Furthermore, a number of alternative embodiments may be formed using the present exemplary system and method. For example, according to one exemplary embodiment, the spherical shaped screw head (110) detailed above may be replaced with a cylindrical member having an engagement orifice (200). According to this exemplary embodiment, the rod connection member may be coupled to or include an integrally formed spherical member configured to be coupled to the engagement orifice (200) via an interference fit, as disclosed above. Additionally, according to various exemplary embodiments, the spherical member of the present system may include a dual-lobed member having a different diameter for each lobe, thereby allowing for provisional and final lock using a corresponding member having a single internal diameter.

As described previously, assemblies detailed herein have a smaller size envelope (e.g., less bulky, lower profile, and/or more compact shape) and are easier to place onto the pedicle screw than traditional designs. The smaller size and ease of installation may reduce trauma to the soft-tissue regions in the vicinity of the surgical site, which in turn generally allows for a quicker recovery by the patient. Yet another possible advantage of the present exemplary pedicle screw systems over existing systems is that all of the parts needed to lock the tulip assembly to the pedicle screw are included within the tulip assembly. Accordingly, once the tulip assembly is coupled to the pedicle screw, no additional member is needed to complete the assembly/installation of the pedicle screw system.

The preceding description has been presented only to illustrate and describe the present method and system. It is not intended to be exhaustive or to limit the present system and method to any precise form disclosed. Many modifications and variations are possible in light of the above teaching.

The foregoing embodiments were chosen and described in order to illustrate principles of the system and method as well as some practical applications. The preceding description enables others skilled in the art to utilize the method and system in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the method and system be defined by the following claims. 

What is claimed and desired to be secured by Letters Patent is:
 1. An implant assembly configured to be coupled to and extend between heads of two pedicle screws, each pedicle screw comprising a head having a head diameter and an elongate threaded portion coupled to and extending from the head, the implant assembly comprising: a first tulip assembly having a first cylindrical bore extending into the first tulip assembly, the first cylindrical bore having a first inner diameter defined by an inner wall of the first tulip assembly, the first inner diameter being smaller than the head diameter of the pedicle screws; a second tulip assembly having a first cylindrical bore extending into the second tulip assembly, the first cylindrical bore having a first inner diameter defined by an inner wall of the second tulip assembly, the first inner diameter being smaller than the head diameter of the pedicle screws; and a rod permanently connected to and extending between the first tulip assembly and the second tulip assembly; wherein the first tulip assembly, the second tulip assembly, and the rod are permanently coupled prior to coupling of the implant assembly to the two pedicle screws.
 2. The implant assembly of claim 1, wherein the first tulip assembly, the second tulip assembly, and the rod are unitarily formed from a single piece of material.
 3. The implant assembly of claim 1, wherein each of the first tulip assembly and the second tulip assembly further comprise a second cylindrical bore comprising: an axis of symmetry shared with an axis of symmetry of the first cylindrical bore of the respective tulip assembly; and an inner diameter smaller than the first inner diameter; wherein the second cylindrical bore extends into the housing from an internal end of the first cylindrical bore.
 4. The implant assembly of claim 1, wherein the first cylindrical bores of the first and second tulip assemblies permit head-first introduction of the head of one of the pedicle screws into the first cylindrical bore until a section having the head diameter is entirely within the first cylindrical bore, thereby creating a compression fit between the inner wall and the head of the pedicle screw within the first cylindrical bore due to the head diameter of the pedicle screw being larger than the inner diameter of the first cylindrical bore.
 5. The implant assembly of claim 4, wherein the compression fit between the inner wall and the head of the pedicle screw is capable of withstanding at least 200 pounds of removal force between the tulip assembly and the pedicle screw along an axis of the bore without relative movement of the screw head along the first cylindrical bore.
 6. The implant assembly of claim 4, wherein each tulip assembly is adapted to permit head-first introduction of the head of the pedicle screw into the first cylindrical bore to the point of creating the compression fit while at least a portion of the elongate threaded portion of the pedicle screw is embedded in bone.
 7. The implant assembly of claim 1, wherein the implant assembly further comprises two pedicle screws and wherein the head of each pedicle screw comprises a truncated spherical shape.
 8. The implant assembly of claim 1, wherein the implant assembly comprises a material selected from the group consisting of a titanium alloy, a cobalt chrome alloy, and a nickel titanium alloy.
 9. An implant assembly comprising: a pedicle screw comprising a head having a head diameter and an elongate threaded portion coupled to and extending from the head; a first tulip assembly and a second tulip assembly, each tulip assembly comprising: a housing defining an engagement orifice and a bore extending into the housing from the engagement orifice, the engagement orifice and the bore having a fixed shape and an inner diameter that is smaller than the head diameter of the pedicle screw; wherein head-first introduction of the head of the pedicle screw through the engagement orifice and into the bore with the screw head in the bore creates a compression fit between the housing and the head of the pedicle screw due to the head diameter of the pedicle screw being larger than the inner diameter of the engagement orifice and the bore; and a rod connected to and extending between the first tulip assembly and the second tulip assembly, wherein the rod is permanently coupled to the two tulip assemblies prior to implantation of the implant assembly by introduction of the pedicle screw into the cylindrical bore of either tulip assembly.
 10. The implant assembly of claim 9, wherein the compression fit between the housing and the head of the pedicle screw is capable of withstanding at least 200 pounds of removal force between the tulip assembly and the pedicle screw along an axis of the bore without relative movement of the screw head along the bore.
 11. The implant assembly of claim 9, wherein the bore is a first bore and the inner diameter is a first inner diameter, and wherein the tulip assembly further comprises a second bore comprising: an axis of symmetry parallel to an axis of symmetry of the first bore; and an inner diameter smaller than the first inner diameter; wherein the second bore extends into the housing from an internal end of the first bore.
 12. The implant assembly of claim 9, wherein the engagement orifice is circular and wherein the inner wall of the housing defines a cylindrical portion of the bore extending into the housing from the engagement orifice.
 13. The implant assembly of claim 9, wherein the head of the pedicle screw comprises a truncated spherical shape.
 14. The implant assembly of claim 9, wherein the first tulip assembly, the second tulip assembly, and the rod are unitarily formed from a single piece of material.
 15. The implant assembly of claim 9, wherein said housing comprises a material selected from the group consisting of a titanium alloy, a cobalt chrome alloy, and a nickel titanium alloy.
 16. A method for affixing a press-on pedicle screw assembly to a spine, comprising: inserting a first pedicle screw having a head portion with a head diameter into a first desired bone of a patient's spine; inserting a second pedicle screw having a head portion with a head diameter into a second desired bone of a patient's spine; after the first and second pedicle screws are inserted into the bones of the patient's spine, guiding a tulip assembly having a housing defining an engagement orifice and a bore extending into the housing from the engagement orifice, the engagement orifice and the bore having a fixed shape and an inner diameter defined by an inner wall of the housing that is smaller than the head diameter of the pedicle screws, until the engagement orifice is proximate the head portion of the first pedicle screw; and applying a force between the first pedicle screw and the tulip assembly whereby the head portion of the pedicle screw is drawn into the bore of the tulip assembly, thereby creating a compression fit between the inner wall of the housing and the head of the pedicle screw due to the head diameter of the pedicle screw being larger than the inner diameter of the engagement orifice and the bore; wherein the tulip assembly is a first tulip assembly of an implant assembly comprising two tulip assemblies connected together by a rod, the connection of the two tulip assemblies to the rod occurring before engaging either tulip assembly to the first pedicle screw.
 17. The method as recited in claim 16 further comprising: positioning a second tulip assembly with its engagement orifice proximate the head portion of the second pedicle screw; and applying a force between the second pedicle screw and the second tulip assembly whereby the head portion of the second pedicle screw is drawn into the bore of the second tulip assembly, thereby creating a compression fit between the inner wall of the housing and the head of the second pedicle screw due to the head diameter of the second pedicle screw being larger than the inner diameter of the engagement orifice and the bore.
 18. The method as recited in claim 16, wherein the first tulip assembly, the second tulip assembly, and the rod are unitarily formed from a single piece of material. 